Plate
tectonics is the same process that causes continents to drift slowly around on
the surface of the Earth, and, says Michelle Selvans, a research geophysicist
at the Smithsonian’s National Air and Space Museum, who wrote a commentary on
the new research for the same journal, “we’ve never seen this anywhere else.”

If plates are indeed
shifting on the Jovian moon, it explains a longstanding mystery. Europa’s
surface is crisscrossed with cracks where the thick ice has spread apart and
the resulting gaps have been filled in by new slushy ice oozing up from the
water deep below. “The fundamental question,” says the paper’s lead author,
University of Idaho planetary scientist Simon Kattenhorn, “is how you can keep
adding new surface without getting rid of old surface?

That’s wouldn’t be a
problem if Europa were simply growing in size, but, writes Selvans, that is
“unlikely.” (She admits privately that this is really science
understatement-speak for “ridiculous.”) It also wouldn’t be a problem if the
old surface simply folded like an accordion, as it was pushed aside. “We’ve
looked for that,” she says, “and haven’t seen it.”

On Earth, however, the
creation of new surface that spreads from places like the submergedMid-Atlantic
ridgeis
balanced by tectonic plates of crustal rock plunging back down to melt in the
sea of magma below. It’s these sinking, melting plates in Earth’s so-called
subduction zones that give rise to volcanoes in the “Ring of Fire”
surrounding the Pacific Ocean.

And now
Kattenhorn and his co-author, Louise Prockter, of Johns Hopkins, seem to have
found evidence that Europa gets rid of its excess crust via subduction as well.
One clue: they looked at surface ice features on Europa that have been
scrambled by repeated cracking and shuffling, then manipulated the imagery to
move the pieces around and reassemble them as they must have been when they
were intact. Some of the puzzle pieces, they discovered, had clearly
disappeared. “We looked at an area about the size of Louisiana,” says
Kattenhorn, “and there was a missing piece the size of Massachusetts.”

Another telltale sign:
along the boundaries where the scientists think some of the crust plunged back
under the adjoining ice, there was evidence of “cryolava”—that is, partially
melted, slushy ice—on one side of the divide but not the other. That’s similar
to what happens on Earth, where volcanoes happen on one side of a subduction boundary
but not the other.

Finally, the existence of
plate tectonics and subduction on Europa would answer another longstanding
question about the frigid moon. Its surface is remarkably deficient in craters
considering the number of comets and asteroids zipping around the neighborhood.

This suggests that Europa
was completely resurfaced no more than 90 million years ago. It could have
happened just that once, but that, says Selvans, feels like “special
pleading”—that we’re looking at the moon at a unique time in its
four-billion-year-plus history. It’s much more palatable to scientists to think
they’re looking at an ongoing process, which plate tectonics certainly is.

Selvans emphasizes that
the evidence so far isn’t a slam-dunk, and Kattenhorn is quick to agree.
Galileo took high-resolution images of only a small part of Europa’s surface.
“Our paper can’t answer the question of whether this is a global process,” he
says. Since melting ice and melting rock behave differently in terms of
buoyancy and density, moreover, it’s not clear that what’s going on at Europa
is an exact analogy for what’s happening on Earth.

The only way to figure it
out for sure is to get more imagery, and Galileo went out of service back in
2003. Unfortunately, the only probe scheduled to visit Europa (and two of
Jupiter’s other moons too) is aEuropean Space Agency mission, which won’t
arrive until 2030. NASA’s own Europa mission, meanwhile, known as theEuropa Clipper, is
still only a concept.